How many different telephone numbers are possible if they must have atleast one '3' and two '7's?
it is a 7 digit phone number.
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What is the length of the phone number in question?
there are 7 digits in the phone number
Well here are the combinations
X represents the variable which is 1,2,4,5,6,8,9,0.
so the permutations would be 7!/2!
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